1. Field of the Invention
This invention relates to a dock leveler which acts as a bridge between a loading dock platform and the bed of carrier and, more particularly, to a pit-mounted dock leveler.
2. Description of the Related Art
Dock levelers for bridging a gap between a loading dock platform and a carrier bed are in widespread use to facilitate the loading and unloading of various types of carriers. Forklift trucks are often required to move from a loading dock directly into and out of the storage compartment of a carrier. In order to facilitate such an operation, dock levelers are used to compensate for any height variation that exists between the loading dock platform and the bed of a carrier.
Typically, a dock leveler will have a main deck which pivots relative to an adjacent loading dock platform and a lip plate which pivots relative to the main deck. The lip plate will rest directly on the bed of a carrier when the dock leveler is positioned as a bridge and will support the main deck at a selected level relative to an adjacent loading dock platform when the dock leveler is in a stored position. Movement of the main deck and lip plate can be achieved by either mechanical means or by fluid pressure.
In a mechanical pit-mounted dock leveler of the type illustrated in FIG. 1, a lift arm assembly provides a constant upward bias on the main deck. These lift arm assemblies typically include a set of four counterbalance springs that must be individually adjusted to provide the desired upward bias on the main deck. A hold-down device is necessary to maintain the main deck at the desired level. When the hold-down device is released, the main deck will automatically be pivoted upwardly by the lift arm assembly to its fully raised position. An operator will then walk down the main dock to the desired level. Once the desired level is reached, the operator steps off the main deck and the hold-down device maintains the main deck at the desired level.
Typically, a mechanical dock leveler includes a lip extension mechanism to pivot the lip plate to an extended bridging position for resting on the bed of a carrier. The lip plate is often counterbalanced to reduce the force necessary to extend the lip plate to the bridging position. Prior art dock levelers have experienced problems in supplying sufficient counterbalancing force to satisfactorily reduce the force necessary to extend the lip plate during the dock leveler opening cycle without hindering the descent, by gravity, of the lip plate during the recycling operation from the extending bridging position to a stored position. In the stored position, the lip plate will rest in keepers provided on the front of the dock leveler frame to support the main deck in a position that is level with the loading dock platform.
Another problem with prior art lip extension mechanisms is that when the dock leveler is cycled and the lip plate is extended to the bridging position, the lip plate will not remain in the extended bridging position. Rather, the lip plate will immediately began to descend to its stored position. The operator will often not have enough time to walk the dock leveler down into engagement with the carrier bed while the lip plate is extended.
A problem with dock levelers that include a pivotably connected main plate and lip plate is that the pivotal connection between these two plates is a weak structural point that limits the load bearing capacity of the leveler. Strengthening this pivotal connection will increase the load bearing capacity of the leveler, thus making it more desirable to the industry. Prior art dock levelers have included a flat bar welded from a hinge tube to the lip plate as a means for strengthening this pivotal connection. However, the flat bar does not provide the necessary support to negate the inherent weakness of the pivotal connection between the main plate and the lip plate.
Another problem with dock levelers is associated with the deck cups that are used to house pull rings attached to chains that release the hold-down mechanism and retract the safety legs for below dock operation. Both the pull ring and the deck cup must be recessed so that they do not interfere with traffic across the main deck. Prior art deck cups typically include a substantially enclosed bottom for capturing the pull ring so that it can be grasped from the top of the main deck. However, such deck cups provide a rough surface that contacts the chain when the pull ring is pulled. This rough surface causes wear on the chain that will eventually necessitate replacement and also causes resistance in the operation of the hold-down mechanism and safety legs. Also, the substantially enclosed deck cups are known to accumulate dirt and other types of debris that must be periodically removed.